The present invention relates to a catalyst for purifying an exhaust gas and a method for purifying an exhaust gas with the catalyst.
Three way catalysts, which can purify HC (hydrocarbon), CO and NOx simultaneously in an exhaust gas in the vicinity of the theoretical air-fuel ratio very effectively, are known as a catalyst for purifying an exhaust gas from an engine. Also known is a so-called lean NOx purifying catalyst, which is as follows. At a lean air-fuel ratio, NOx contained in the exhaust gas is stored in a NOx storage material such as Ba, and at the theoretical air-fuel or a rich air-fuel ratio, the stored NOx is migrated onto a precious metal and is reacted with a reducing gas such as HC, CO and H2 contained in the exhaust gas to reduce NOx to N2 for purification, and to oxidize and purify the reducing gases at the same time.
Generally, these catalysts contain an oxygen storage material that stores and releases oxygen by changing the oxidation number, and CeO2 or a CeO2xe2x80xa2ZrO2 mixed oxide is commonly used as the oxygen storage material. In the three way catalysts, these oxides serve to correct a deviation from the theoretical air-fuel ratio by storing or releasing oxygen. In the lean NOx purifying catalyst, these oxides serve as an oxygen supply source for oxidizing a large amount of NO contained in an exhaust gas to NO2, which can easily be stored in the NOx storage material.
Japanese Patent Laid-Open Publication NO.6-315634 discloses a catalytic structure for nitrogen-oxygen catalytic reduction comprising a carrier, an inner layer on the carrier and a surface layer on the inner layer, the inner layer comprising a catalytic component expressed by a general formula AxB1-xCO3, (where A is at least one element selected from the group consisting of La, Y, Ce and the like, B is at least one element selected from the group consisting of Na, K, Sr and the like, and C is at least one element selected from the group consisting of Mn, Co, Zr and the like, and 0xe2x89xa6xc3x97xe2x89xa61), the surface layer comprising a catalyst component where an active component comprising an oxide of an element of Group Ib, IIa or IIb of the periodic table is supported by a support such as aluminum oxide, titanium dioxide, and zirconium oxide. In this catalytic structure, hydrocarbon is adsorbed by the surface layer for activation, whereas a nitrogen oxide is adsorbed onto the inner layer for activation, so that the activated hydrocarbon and the activated nitrogen oxide are reacted at the interface therebetween. High activity and selectivity for reduction of the nitrogen oxide are expected from this structure.
However, when a catalyst is exposed to a high temperature atmosphere for a long time, an oxide constituting the oxygen storage material is deteriorated, so that oxygen is not stored or released properly. As a result, the performance of the catalyst for purifying an exhaust gas is deteriorated.
The object of the present invention is to provide a catalyst for purifying an exhaust gas whose performance for purifying an exhaust gas is not significantly deteriorated at exposure to a high temperature atmosphere for a long time and that has excellent heat resistance.
Another object of the present invention is to improve the sulfur poisoning resistance and the regeneration properties from sulfur poisoning.
Furthermore, another object of the present invention is to provide a method for purifying an exhaust gas using such a catalyst.
The present invention uses a Cexe2x80x94Zrxe2x80x94Sr mixed oxide containing Ce, Zr and Sr as constituent elements (which may be referred to as a CeO2xc2x7ZrO2xc2x7SrO mixed oxide in the following description).
A catalyst for purifying an exhaust gas of the present invention includes a catalytic metal that serves for oxidation of HC and CO in the exhaust gas containing oxygen and reduction of NOx in the exhaust gas, and a mixed oxide comprising Ce, Zr and Sr.
According to this embodiment, the mixed oxide that acts as an oxygen storage material contains Sr in addition to Ce and Zr, so that the oxygen storage function of the catalyst is not significantly deteriorated even if the catalyst is exposed to a high temperature atmosphere for a long time. Moreover, a catalyst having an excellent heat resistance can be obtained. The reason for this is not clear, but it may be as follows.
An analysis indicates that when a Cexe2x80x94Zr mixed oxide (CeO2xc2x7ZrO2 mixed oxide) is heated, ZrO2 is separated. However, in a Cexe2x80x94Zrxe2x80x94Sr mixed oxide, such separation of ZrO2 hardly occurs, and the Cexe2x80x94Zrxe2x80x94Sr mixed oxide is highly crystalline. Therefore, even if it is exposed to a high temperature, it hardly is degraded so that the oxygen storage function is not deteriorated. It seems that Sr contributes to this high crystallinity.
An analysis indicates that since the primary particles of the Cexe2x80x94Zrxe2x80x94Sr mixed oxide have a small particle size, it is difficult that sintering due to heat proceeds. It seems that Sr contributes to the fact that the particles are fine.
An analysis indicates that since the secondary particles of the Cexe2x80x94Zrxe2x80x94Sr mixed oxide have a large particle size, mesopore is also large which makes it easy that the exhaust gas is diffused to the inside. This advantageously serves for storage and release of oxygen, and therefore high oxygen storage ability can be exerted at a relatively high temperature as well. Furthermore, it seems that the fact that Sr activates oxygen advantageously serves for storage and release of oxygen.
Therefore, the catalyst of the present invention can be provided at an exhaust pipe in which the temperature of the catalyst is constantly or temporarily at 700xc2x0 C. or more or in a place where the temperature of the catalyst is at 800xc2x0 C. or more, or at further higher 900xc2x0 C. or more, such as a site immediate downstream of an exhaust manihold.
Furthermore, when the present invention is used to a ternary catalyst, even if the catalyst is exposed to a high temperature atmosphere for a long time, the mixed oxide effectively can function as the oxygen storage material that corrects a deviation of the air-fuel ratio from the theoretical air-fuel ratio by storage and release of oxygen. Thus, high HC purification performance can be obtained.
When the present invention is used as a lean NOx purifying catalyst, even if the catalyst is exposed to a high temperature atmosphere for a long time, the mixed oxide effectively can function as the supply source that supplies oxygen having high activity for oxidation of NO. Thus, at a lean air-fuel ratio, NO is oxidized to NO2, which is readily stored by the NOx storage material, so that high lean NOx purification performance can be obtained.
NOx storage materials have a problem of so-called sulfur poisoning that deprives a NOx storage material of its function as a NOx storage material, because of its formation of a salt by reacting with a sulfur oxide contained in exhaust gases. However, the Cexe2x80x94Zrxe2x80x94Sr mixed oxide makes it possible that deterioration of the lean NOx purification performance due to sulfur poisoning can be suppressed to a small level, and provides a catalyst having excellent sulfur poisoning resistance. The reason for this is not clear, but it seems that the presence of Sr provides fine particles of the NOx storage materials and the surface area of the NOx storage materials becomes large, so that the NOx storage materials are unsusceptible to sulfur poisoning. In addition, the catalyst can be regenerated by raising the temperature of the sulfur-poisoned catalyst, and in the catalyst of this embodiment, the Cexe2x80x94Zrxe2x80x94Sr mixed oxide has high heat resistance so that very high regeneration performance can be obtained.
The Cexe2x80x94Zrxe2x80x94Sr mixed oxide is advantageous for sulfur poisoning resistance when the content of Zr becomes large, and the heat resistance thereof is improved when the content of Ce becomes large. However, the content of Sr is too excessive, the heat resistance is deteriorated.
In the Cexe2x80x94Zrxe2x80x94Sr mixed oxide, the amount of released oxygen is not very large when the air-fuel ratio of the engine is the stoichiometric or rich ratio at a regular temperature of the exhaust gas of around 350xc2x0 C. Therefore, the period of time during which the air-fuel ratio is kept stoichiometric or rich to release NOx absorbed in the NOx storage materials for reduction and purification can be shortened, or the degree of the rich ratio can be reduced.
More specifically, when the amount of released oxygen is large, even if the reduction components (HC, CO, H2, etc.) in the exhaust gas for purification of NOx is made large by making the air-fuel ratio stoichiometric or rich, the amount of the reduction components consumed by reaction with the released oxygen also becomes large. Therefore, a larger amount of reduction components is required for reduction and purification of NOx. In other words, it is necessary to prolong the period of time during which the air-fuel ratio is kept stoichiometric or rich or to raise the degree of the rich ratio. On the other hand, the Cexe2x80x94Zrxe2x80x94Sr mixed oxide has a small amount of released oxygen, so that the amount of consumed reduction components is small. Therefore, the period of time during which the air-fuel ratio is kept stoichiometric or rich for reduction and purification of NOx can be shortened, or the degree of the rich ratio can be reduced. Consequently, the amount of fuel consumption for the stoichiometric or rich air-fuel ratio can be reduced.
A method for producing the Cexe2x80x94Zrxe2x80x94Sr mixed oxide may be, but not limited to, coprecipitation in which alkali is dropped to a mixed aqueous solution comprising salts of Ce, Zr and Sr dissolved to precipitate a mixed oxide; a solid phase reaction in which a mixture of particles of oxides of Ce, Zr and Sr is melted at a high temperature to produce a mixed oxide; evaporation to dryness in which an aqueous solution containing ions of one or two metals of Ce, Zr and Sr is prepared, oxide powder of the remaining metal of Ce, Zr and Sr is placed in the aqueous solution, the aqueous solution is stirred, and the aqueous solution is dried and calcined to form a mixed oxide; a method for obtaining crystals of a mixed oxide by boiling a mixed solution comprising salts of Ce, Zr and Sr dissolved to remove water; or the like.
When a precious metal is used as the catalytic metal, activated oxygen is supplied from the mixed oxide, and NOx and HC in the exhaust gas can be activated on the surface of the precious metal. Therefore, an oxidation reaction of NO in the exhaust gas to NO2, and partial oxidation reaction of HC proceeds smoothly. Since NO2 and the partially oxidized HC are highly reactive in terms of energy, reduction of NOx and oxidation of HC proceed efficiently.
The catalyst of the present invention can contain a NOx storage material that absorbs NOx in the exhaust gas in an oxygen-excessive atmosphere in which an oxygen concentration in the exhaust gas is high (lean air-fuel ratio), and releases the absorbed NOx by reduction of the oxygen concentration (rich air-fuel ratio). Thus, the catalyst can act as a so-called lean NOx purifying catalyst. In this case, as described above, the mixed oxide effectively can function as a supply source that supplies oxygen for oxidation of NO, even if the catalyst is exposed to a high temperature atmosphere for a long time. Therefore, the present invention provides high lean NOx purification performance by oxidizing NO to NO2 and absorbing the NO2 in the NOx storage materials at a lean air-fuel ratio.
A specific embodiment of such a lean NOx purifying catalyst includes a carrier; an inner catalytic layer disposed on the carrier containing a precious metal, a NOx storage material and the Cexe2x80x94Zrxe2x80x94Sr mixed oxide; and an outer catalytic layer disposed on the inner catalytic layer containing a precious metal and zeolite, the inner catalytic layer and the outer catalytic layer being laminated on the carrier in this order.
According to this embodiment, at a lean air-fuel ratio, in the outer catalytic layer, HC that has been stored in zeolite is released and reacted with NO in the exhaust gas for purification of NOx. In the inner catalytic layer, NO2 generated by oxidation of NO in the outer catalytic layer is stored in the NOx storage materials, and apparently NOx is purified. NO2 stored in the NOx storage materials is reacted with activated partially oxidized HC on the precious metals of the outer catalytic layer when the air-fuel ratio is turned to be rich so that NO2 is degraded and purified. These effects of the two layers are combined so that very high lean NOx purification performance can be exhibited. Therefore, the outer catalytic layer exerts a function as a catalyst for selective reduction NOx purification, and the inner catalytic layer exerts a function as a catalyst for lean reduction NOx purification.
As the NOx storage material, it is preferable to use a combination of Ba, K, Sr, and Mg. Thus, deterioration of the NOx absorption ability of the NOx storage material due to sulfur poisoning can be suppressed. Moreover, the heat resistance of the NOx storage material can be improved. The reason for this is not clear, but it seems as follows.
First, it seems that the elements (K, Sr, Mg) other than Ba are more susceptible to sulfur poisoning than Ba, so that the degree of sulfur poisoning of Ba is made relatively small. More specifically, Ba has higher NOx absorption ability than those of the other elements, but the presence of the other elements makes the degree of sulfur poisoning of Ba relatively small. Therefore, the degree of decrease of the NOx absorption ability is small.
According to an analysis, it appears that Ba and Sr (at least a part of each of them) form a compound (a mixed oxide or a double salt) constituted by these two elements. It seems that such a Baxe2x80x94Sr compound is less susceptible to sulfur poisoning than Ba alone, so that deterioration of the NOx absorption ability can be suppressed.
According to an analysis, it appears that Ba and Mg (at least a part of each of them) come close to each other or are combined to be nearly amorphous, although it is not crystalline. Such a Baxe2x80x94Mg coexisting substance suppresses sulfur poisoning of Ba (production of barium sulfate) more than in the case of Ba alone, so that deterioration of the NOx absorption ability can be suppressed.
According to an analysis, it appears that K is not combined with or not be compatible with Ba, Sr or Mg, and is dispersed around the Baxe2x80x94Sr compound or the Baxe2x80x94Mg coexisting substance. It seems that since K is relatively highly reactive with sulfur, K prevents the Baxe2x80x94Sr compound or the Baxe2x80x94Mg coexisting substance from being sulfur-poisoned. Furthermore, K facilitates crystallinity of the Baxe2x80x94Sr double carbonate, and activates the NOx storage material. Therefore, K contributes to improvement of the heat resistance of the catalyst.
It seems that the quaternary material of Baxe2x80x94Kxe2x80x94Srxe2x80x94Mg as the NOx storage material has weakened bonding to SOx because of an interaction between the four elements, so that even if SOx binds thereto, it can be detached readily.
When Ba is an only element constituting the NOx storage material and the amount thereof is increased, only the particle size is increased, and the specific surface area is not significantly increased. However, when Ba is combined with the other elements (K, Sr, Mg) and the amount thereof is increased, the particle size is not significantly increased, and the specific surface area or the active site is increased. Therefore, it seems that the volume of absorbed NOx and SOx is increased. Therefore, even if more or less sulfur poisoning occurs, the NOx absorption ability is not significantly deteriorated.
As described above, the combination of Ba and the other elements (K, Sr, Mg) is advantageous to provide fine particles of the NOx storage materials. In particular, Sr has a significant function to make Ba and Mg particles fine. Thus, high dispersibility on the support of the NOx storage materials can be achieved, and heat sintering hardly occurs. In other words, the heat resistance of the catalyst can be high.
When the Cexe2x80x94Zrxe2x80x94Sr mixed oxide and alumina are used together as the NOx storage material and the support material of the precious metal, the heat deterioration of the catalyst advantageously can be prevented, because the alumina hardly is sintered or broken even at a high temperature. However, in the case of alumina, when the catalyst has a high temperature, Ba is reacted with the support and this facilitates deterioration. On the other hand, Mg serves to suppress the reaction of the support and Ba and prevents the heat deterioration of the catalyst.
As the alumina, for a ternary catalyst, an addition alumina added with Ba, Zr, La, or the like to suppress reduction of the specific surface area when the catalyst is exposed to high temperatures may be used. However, it is advantageous to use a non-addition alumina that does not contain these additional elements for NOx purification at a lean ratio. More specifically, at a lean ratio, the precious metal acts as a catalyst for oxidizing NO in the exhaust gas to NO2, and assists the absorption of NOx by the NOx storage materials. The alumina serves to assist the catalytic reaction of the precious metal. When an additive as described above is present, the function of the alumina as a cocatalyst is deteriorated, although the heat resistance is improved. Therefore, a non-addition alumina is advantageous for NOx purification at a lean ratio.
It is preferable to combine alumina and a Cexe2x80x94Zrxe2x80x94Sr mixed oxide at a mass ratio of 1:1 or more or less 1:1. This is advantageous for both improvement of the heat resistance of the catalyst and improvement of the sulfur poisoning resistance.
As the precious metal, it is preferable to use Pt, which has a high catalytic function for oxidation of NO to NO2 at a lean ratio and reduction of NO2 to N2 at the stoichiometric or rich ratio. It is more preferable to use both Pt and Rh. Rh serves to assist a catalytic reaction of Pt, namely, promotes the ternary reaction described above at the stoichiometric or rich ratio, and promotes a reduction and degradation reaction of NOx released from the NOx storage material. When the Rh support amount per L of the carrier is in the range from about 0.1 to 1.0 g, the Rh support amount does not significantly affect the NOx purification ratio. Therefore, the Rh support amount can be small.
It is preferable that the Pt support amount per L of the carrier is 1 to 15 g. Amounts of less than 1 g do not allow sufficient reduction and purification. Amounts of more than 15 g provide no improvement in the NOx purification ratio, leading to high cost. The Rh support amount is preferably, for example, about 1/10 to 1/100 of the Pt support amount.
In the catalyst for purifying an exhaust gas, the support amount of Sr as the NOx storage material per L of the carrier is preferably 8 to 20 g, and the support amount of Mg per L of the carrier is preferably 5 to 15 g, more preferably, 8 to 12 g.
Thus, the effect of Mg on improvement of the heat resistance can be obtained, and at the same time, the effect of Mg and Sr on improvement of the sulfur poisoning resistance can be obtained. The Ba support amount per L of the carrier is 25 to 60 g.
In the catalyst for purifying an exhaust gas, the mass ratio of Ba, Sr and Mg in the catalytic layer is preferably Ba:Sr: Mg=30:(8 to 20):(8 to 12).
This is advantageous for improvement of the heat resistance of the NOx storage material while suppressing the sulfur poisoning of the NOx storage material.
In the catalyst for purifying an exhaust gas, the mass ratio of Ba, K, Sr and Mg in the catalytic layer is preferably Ba:K: Sr:Mg=30:(2 to 12):(8 to 20):(8 to 12).
This is advantageous for improvement of the heat resistance of the NOx storage material while suppressing the sulfur poisoning of the NOx storage material.
In the catalyst for purifying an exhaust gas, the support amount of K per L of the carrier is preferably 2 to 12 g.
More specifically, promotion of the crystallinity of the Baxe2x80x94Sr double carbonate by K and the resulting improvement of the heat resistance of the catalyst can be achieved when the K support amount is 2 g/L or more. However, when the K support amount exceeds 12 g/L, the effects are weakened. In this case, the K support amount is more preferably 4 to 10 g/L.
In the catalyst for purifying an exhaust gas, the support amount of K per L of the carrier is preferably 2 to 6 g.
More specifically, since the support amount of K per L of the carrier is 6 g or less in the present invention, deterioration of the oxidation and purification ability of HC due to the precious metal can be suppressed when the oxygen concentration in the exhaust gas decreases after exposed to a high temperature atmosphere (when an atmosphere with reductants (xcexxe2x89xa61) is reached).
Since the support amount of K per L of the carrier is 2 g or more in the present invention, the effect of K on preventing sulfur poisoning of Ba, Mg and Sr can be obtained, and NOx released from the NOx storage material can be reacted with HC sufficiently for purification when switching a lean combustion operation to a theoretical air-fuel ratio combustion operation or a rich combustion operation.
When the support amount of K per L of the carrier is 2 g to 6 g, the mass ratio of Ba and K in the catalytic layer is preferably Ba:K=(5 to 15):1.
More specifically, since the mass ratio of the Ba support amount to the K support amount is 5 or more, the NOx absorption ability never becomes insufficient, which might occur when the Ba support amount is small. Since this mass ratio is 15 or less, the NOx absorption site of Ba never decreases, which might be caused by sintering during catalyst calcining because the Ba support amount is large. Moreover, there is no occurrence of detachment of Ba as a result of crystallization of Ba on the support.
Therefore, when the oxygen concentration in the exhaust gas is high (during lean combustion operation of the engine), the NOx absorption properties of Ba are not deteriorated. When the oxygen concentration in the exhaust gas becomes low (during theoretical air-fuel ratio combustion or rich combustion operation of the engine), the NOx released from Ba can be reacted with HC sufficiently. Thus, such a function can be performed more properly.
In the catalyst for purifying an exhaust gas, xe2x80x9cwhen the oxygen concentration in the exhaust gas is highxe2x80x9d refers to, for example, when the oxygen concentration is at least 5%.
In the catalyst for purifying an exhaust gas, the engine can be a gasoline engine for lean burning or a diesel engine.
The catalyst for purifying an exhaust gas that is provided in the passage of exhaust gases from the engine and reduces the NOx concentration in the exhaust gas containing NOx, sulfur and oxygen can be produced by a method comprising the steps of:
coating a carrier with a Cexe2x80x94Zrxe2x80x94Sr mixed oxide and alumina as a support; and
impregnating the coating layer with a Ba solution, a K solution, a Sr solution, a Mg solution and a solution of a precious metal.
This embodiment provides a catalyst for purifying an exhaust gas comprising a carrier and a catalytic layer on the carrier, the catalytic layer comprising Ba, K, Sr and Mg as NOx storage materials and a precious metal for reducing NOx that are supported on a support (Cexe2x80x94Zrxe2x80x94Sr mixed oxide and alumina). Thus, the heat resistance of the NOx storage material can be improved while suppressing sulfur poisoning of the NOx storage materials.
In the method for producing the catalyst for purifying an exhaust gas, all of the Ba solution, the K solution, the Sr solution, and the Mg solution are preferably acetate solutions.
In the method for producing the catalyst for purifying an exhaust gas, it is preferable to form the coating layer in the form of a multiple layer by coating the carrier with the support by two operations, and then impregnating the two layers with the Ba solution, the K solution, the Sr solution, the Mg solution, and the precious metal solution.
More specifically, in forming a thick catalytic layer on a carrier, when the carrier is coated with the support by one operation, the thickness of the support layer tends to be non-uniform because the amount of the support is large. In addition, drying and calcining of the support layer takes a long time. On the other hand, coating by two operations, as described above, is advantageous for achieving a uniform thickness of the support layer, and time for drying and calcining can be shortened. Furthermore, when the support layer is constituted by two layers, and the support layer is impregnated with the NOx storage materials, the concentration of the NOx storage materials in the outer support layer is higher than that of the inner support layer. Therefore, SOx is trapped primarily by the NOx storage materials of the outer support layer, and this ensures that the inner support layer can be provided with the NOx storage materials that are sulfur-poisoned only in a small level. This is advantageous for maintaining the NOx purification performance.
In the method for producing the catalyst for purifying an exhaust gas, it is preferable to mix the Ba solution, the K solution, the Sr solution, the Mg solution and the solution of a precious metal so that the support is impregnated with the solutions simultaneously.
More specifically, when the solution of a precious metal solution and the solution of the NOx storage materials are separated and the support is impregnated with the solution of a precious metal first, the precious metal is covered by the NOx storage materials used later for impregnation, and tends to be buried therein. On the other hand, when the support is impregnated with the solution of a precious metal later, the NOx storage materials, especially Ba, are eluded in the solution of a precious metal so that the dispersibility becomes poor.
On the other hand, simultaneous impregnation allows the precious metal to be arranged close to the NOx storage materials without the precious metal being buried. In addition, simultaneous impregnation does not lead to poor dispersibility of Ba, so that this is advantageous for NOx reduction and purification. Furthermore, simultaneous impregnation of four kinds of NOx storage material solutions efficiently forms the Baxe2x80x94Sr compound and the Baxe2x80x94Mg coexisting substance and allows K to be dispersed around them. This is advantageous for suppressing sulfur poisoning of the NOx storage materials, and for providing fine particles of the NOx storage materials, in particular, fine particles of Ba and Mg provided by an action of Sr. Thus, the heat resistance of the catalyst becomes high.
In the method for producing the catalyst for purifying an exhaust gas, when the Ba solution, the K solution, the Sr solution, the Mg solution are divided into two groups, one for the earlier impregnation of the support layer and one for the later impregnation, it is preferable to use the K solution in the later impregnation.
More specifically, in the case where the Ba solution, the K solution, the Sr solution, and the Mg solution are used to impregnate the support simultaneously, when the amounts of Ba, K, Sr and Mg to be supported are large, the concentrations of the metals in the impregnation solution become high, and therefore for example Ba, which has a low solubility, remains in the impregnation solution without being dissolved. In this case, in the metal components are non-uniform in impregnation, so that the catalyst performance may be reduced.
On the other hand, if the impregnation solution is heated, the solubility is increased so that all the metal components can be dissolved without increasing the total amount of the impregnation solution. However, the heating process is required. Therefore, it is preferable that the Ba solution, the K solution, the Sr solution, the Mg solution are divided into two groups, one for the earlier impregnation of the support and one for the later impregnation, and the K solution is used in the later impregnation.
In this case, since K is not combined with or not compatible with the other NOx storage materials, it is not necessary to use the K solution for impregnation at the same time with the other NOx storage materials. On the contrary, using the K solution in the later impregnation is advantageous
In the method for producing the catalyst for purifying an exhaust gas, when the Ba solution, the K solution, the Sr solution, the Mg solution are divided into two groups, one for the earlier impregnation of the support and one for the later impregnation, it is preferable to use the Sr solution in the earlier impregnation.
More specifically, since it seems that Sr serves to make the particles of Ba and Mg fine, the particles of Ba and Mg are made fine by Sr being supported earlier, which is advantageous for enhancing the heat resistance of the catalyst.
Furthermore, an apparatus for purifying an exhaust gas can be constructed as shown in FIG. 1. More specifically, the apparatus includes:
a catalyst 25 for purifying an exhaust gas provided in a passage 22 for an exhaust gas from an engine 1 or the like, comprising a NOx storage material that absorbs NOx and a sulfur component in the exhaust gas in an oxygen-excessive atmosphere in which an oxygen concentration in the exhaust gas is high, and releases the absorbed NOx by reduction of the oxygen concentration;
sulfur-excessive absorption determining means a for determining a excessive absorption state of the sulfur component in the NOx storage material; and
sulfur detaching means b for detaching the sulfur component from the NOx storage material by raising the temperature of the catalyst 25 and lowering the concentration of oxygen, when the sulfur-excessive absorption determining means a determines that the absorption of the sulfur component is in an excessive state.
wherein the NOx storage material is constituted by Ba and at least one element selected from the group consisting of K, Sr, Mg and La.
In such an embodiment, the sulfur detaching means b is operated after the sulfur component (SOx) in the exhaust gas has been absorbed in the NOx storage material excessively. This embodiment makes it easy to regenerate the NOx storage material almost to the NOx absorption ability before the sulfur component is absorbed. In other words, the NOx absorption ability of the NOx storage materials after regeneration (which means regeneration from sulfur poisoning, which also applies to the following) is higher than that the NOx storage material comprising Ba alone, or the degree of deterioration of the NOx absorption ability when exposed to a high temperature is smaller. In other words, the heat resistance is higher. This improvement of the heat resistance is advantageous for regeneration of the NOx storage materials. The relationship between the improvement of the heat resistance and the regeneration of the NOx storage materials is as follows.
The sulfur detaching means b detaches the sulfur component from the NOx storage materials not only by lowering the concentration of oxygen in the exhaust gas, but also by raising the temperature of the catalyst 25. Therefore, for a catalyst comprising NOx storage materials having a low heat resistance, it is difficult to raise the temperature of the NOx storage materials to detach the sulfur component, which prevents achievement of the original object of the present invention. On the other hand, as in the present invention, when the heat resistance of the NOx storage materials is high, the sulfur detaching means b can be effectively used for regeneration of the NOx absorption ability. In other words, deterioration of the NOx storage materials due to heat during sulfur detaching treatment can be avoided.
Thus, the NOx absorption ability after regeneration is higher than that the NOx storage material comprising Ba alone, or the heat resistance is higher. The reason for this is not clear, but it seems to be as follows.
First, it seems that the elements (K, Sr, Mg or La) other than Ba are more susceptible to sulfur poisoning than Ba, so that the degree of sulfur poisoning of Ba is made relatively small. Therefore, the degree of a decrease of the NOx absorption ability after sulfur poisoning is small. More specifically, Ba has higher NOx absorption ability than those of the other elements, but the presence of the other elements makes the degree of sulfur poisoning of Ba relatively small. Therefore, the degree of decrease of the NOx absorption ability is small.
Furthermore, it seems that the elements (K, Sr, Mg or La) other than Ba are more readily to be regenerated from sulfur poisoning than Ba, so that the NOx absorption ability after regeneration is higher. In other words, a sulfate in which Ba is combined with SOx is stable. However, sulfates of the other elements is unstable compared with the sulfate of Ba, and therefore, SOx can be easily detached in an atmosphere at a high temperature and a low oxygen concentration.
Furthermore, it seems that Ba is combined with the other elements (Sr, Mg or La) except K (forming a mixed oxide or a double salt, or being close or binding to each other to be nearly amorphous), which makes it difficult for sulfur poisoning to occur.
Furthermore, when the NOx storage material is constituted only by Ba, and the amount thereof is increased, the NOx absorption ability before sulfur poisoning and after regeneration is not significantly improved. This seems to be because when the amount of Ba exceeds a certain amount, only the particle size is increased, and the specific surface area is not increased. However, when Ba is combined with the other elements (at least one selected from K, Sr, Mg and La), each is present separately because of the difference in the nature between the elements, and the specific surface area or the active site is increased. In addition, sintering due to heat hardly occurs. Furthermore, the interaction between the different elements constituting the NOx storage materials facilitate the detachment of the sulfur component.
As described above, the combination of Ba and the other elements (at least one selected from K, Sr, Mg and La) is advantageous to provide fine particles of the NOx storage materials. In particular, Sr has a significant function to make Ba and Mg particles fine. Thus, high dispersibility on the support of the NOx storage materials can be achieved, and heat sintering hardly occurs. In other words, the heat resistance of the catalyst can be high.
When the support is alumina, Ba is reacted with the support when the catalyst reaches at a high temperature, which is likely to lead to deterioration. However, Mg serves to suppress the reaction between the support and Ba, so that the heat resistance of the catalyst can be high.
When Ba and the other elements (at least one selected from K, Sr, Mg and La) is supported by a carrier having a honeycomb shape or other shapes, the Ba support amount per L of the carrier is preferably about 10 to 50 g, more preferably 20 to 40 g. The support amounts of the other elements are preferably equal to or less than the support amount of Ba.
The exhaust gas with excessive oxygen having a high concentration of oxygen corresponds to an exhaust gas (a concentration of oxygen of about 4 to 20%) when the engine is operated in a lean air-fuel mixture having an air-fuel ratio A/F greater than 16 (in particular, A/F=18 to 50).
It is preferable that the elements constituting the NOx storage materials include K in addition to Ba. This achieves a high NOx absorption ability before sulfur poisoning. Furthermore, K is not combined with Ba, but is highly reactive with sulfur, so that K is present around Ba and prevents Ba from sulfur-poisoned, and suppresses deterioration of the NOx absorption ability due to sulfur poisoning of Ba. Furthermore, it seems that K is more readily to detach the sulfur component than Ba, so that the NOx absorption ability after regeneration is higher. The mass ratio of Ba and K is preferably, for example, Ba:K=30: (1 to 30).
It is preferable that the elements constituting the NOx storage materials include at least one selected from Sr, Mg and La, in addition to Ba and K. This is advantageous for a high heat resistance of the NOx storage materials and prevention of heat deterioration during sulfur detachment treatment.
According to an analysis, it appears that Ba and Sr (at least a part of each of them) form a compound (a mixed oxide or a double salt) constituted by these two elements. It seems that such a Baxe2x80x94Sr compound is less susceptible to sulfur poisoning than Ba alone, so that deterioration of the NOx absorption ability can be suppressed.
According to an analysis, it appears that Ba and Mg (at least a part of each of them) come close to each other or are combined to be nearly amorphous, although it is not crystalline. Such a Baxe2x80x94Mg coexisting substance suppresses sulfur poisoning of Ba more than in the case of Ba alone, so that deterioration of the NOx absorption ability can be suppressed.
According to an analysis, it appears that K is not combined with or not be compatible with Ba, Sr or Mg, and is dispersed around the Baxe2x80x94Sr compound or the Baxe2x80x94Mg coexisting substance. It seems that since K is relatively highly reactive with sulfur, K prevents the Baxe2x80x94Sr compound or the Baxe2x80x94Mg coexisting substance from being sulfur-poisoned.
When Ba, K and Mg are used as the elements constituting the NOx storage materials and are supported by a carrier having a honeycomb or other shapes, the Ba support amount per L of the carrier is preferably 10 to 50 g, the K support amount is preferably 1 g (the upper limit is 15 g, for example), and the Mg support amount is preferably 3 to 17 g. For the Mg support amount, an amount of 5 to 15 g is more preferable, and an amount of 8 to 12 g is even more preferable. These amounts provide high heat resistance and good regeneration properties from sulfur poisoning. The mass ratio of Ba, K and Mg is preferably, for example, Ba:K:Mg=30: (1 to 30):(1 to 30).
When Ba, K and Sr are used as the elements constituting the NOx storage materials and are supported by a carrier having a honeycomb or other shapes, a preferable Ba support amount and a preferable K support amount per L of the carrier are the same as those in the Baxe2x80x94Kxe2x80x94Mg based catalyst. The Sr support amount is preferably 10 to 20 g. For the Sr support amount, 13 to 17 g are more preferable. These amounts provide high heat resistance and good regeneration properties from sulfur poisoning. The mass ratio of Ba, K and Sr is preferably, for example, Ba:K:Sr=30: (1 to 30):(1 to 30).
It is preferable that the elements constituting the NOx storage materials include Sr, in addition to Ba. This is advantageous for achievement of a high heat resistance of the NOx storage materials and prevention of heat deterioration during sulfur detachment treatment.
It is preferable that the elements constituting the NOx storage materials include at least one selected from Mg and La, in addition to Ba and Sr. This is more advantageous for achievement of a high heat resistance of the NOx storage materials and prevention of heat deterioration during sulfur detachment treatment.
It is preferable that the elements constituting the NOx storage materials include Mg, in addition to Ba. This is advantageous for achievement of a high heat resistance of the NOx storage materials and prevention of heat deterioration during sulfur detachment treatment.
It is preferable that the elements constituting the NOx storage materials include La, in addition to Ba and Mg. This is more advantageous for achievement of a high heat resistance of the NOx storage materials and prevention of heat deterioration during sulfur detachment treatment.
Raising the temperature of the catalyst 25 by the sulfur detaching means b can be achieved by raising the temperature of the exhaust gas. For example, a temperature of the exhaust gas of 500 to 1100xc2x0 C. (preferably 600 to 1100xc2x0 C.) is preferable for detachment of sulfur from the NOx storage materials. A heater can be provided in the catalyst 25 and can be heated. Reducing the concentration of oxygen in the exhaust gas by the sulfur detaching means b can be achieved by controlling the air-fuel ratio of the engine. For example, xcex (oxygen-excessive ratio) of around 1 or not more than 1 achieves a concentration of oxygen in the exhaust gas of 0.5% or less, and further leads to an increase in the amount of the reduction components such as HC, CO, H2 or the like in the exhaust gas. This is advantageous for detachment of the sulfur component from the NOx storage materials.
When a spark ignition direct injected engine is used as the engine, the sulfur detaching means b is preferably fuel injection control means that operates a fuel injection valve in such a manner that fuel is divided into at least two portions to be injected to the combustion chamber in the cylinder during a period from the start of an air-intake stroke to the end of the compression stroke. This makes it possible to raise the temperature of the catalyst 25 by raising the temperature of the exhaust gas while reducing the concentration of oxygen in the exhaust gas. If such a divisional injection is used, in particular, the concentration of Co in the exhaust gas can be increased, which is more advantageous for detachment of the sulfur component from the NOx storage materials.
More specifically, it seems that when the NOx storage material is Ba, SOx is adsorbed onto the surface of barium particles in the form of a sulfate, and the barium sulfate generates barium carbonate and sulfur dioxide by the following reaction progress by supply of CO.
BaSO4+CO xe2x86x92BaCO3+SO2↑(coefficients omitted)
Furthermore, when the CO concentration becomes high, a so-called water gas shift reaction proceeds between CO and water in the exhaust gas, thereby generating hydrogen in the reaction site of the catalyst.
CO+H2O xe2x86x92H2+CO2Then, the action of hydrogen causes the sulfur component adsorbed onto the NOx storage material to be detached. This is advantageous for detachment of the sulfur component. Since the water gas shift reaction proceeds even in a relatively low temperature, it is not necessary to raise the temperature of the catalyst 25.
The sulfur excessive absorption determining means that determines the excessive absorption state of the sulfur component to the NOx storage material operates in the following manner, for example: estimating an amount of absorbed SOx in the NOx storage material, based on the travel distance of the automobile and the total amount of fuel consumed during that period, or further in view of the temperature of the catalyst 25 during that period, and determining that the sulfur component reaches the excessive absorption state when the estimated value exceeds a predetermined value.
Therefore, a method for purifying an exhaust gas including NOx and a sulfur component preferably includes:
allowing a NOx storage material comprising Ba and at least one selected from the group consisting of K, Sr, Mg and La to absorb the NOx and the sulfur component by contacting the exhaust gas with the NOx storage material when the exhaust gas is in an oxygen-excessive state in which an oxygen concentration is high, and
raising the temperature of the NOx storage material and reducing the concentration of oxygen in the exhaust gas when the sulfur component absorption state of the NOx storage material reaches a predetermined excessive absorption state, thereby detaching the sulfur component from the NOx storage material.
As seen from the above description, such a method facilitates detachment of the sulfur component from the NOx storage material to recover the NOx absorption ability to a high level, when the NOx absorption ability of the NOx storage material is deteriorated by sulfur poisoning. Thus, this method is advantageous for purification of NOx.
This and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.